Semiconductor memory chips such as DRAMs commonly include both the memory itself and additional peripheral circuits to perform various functions.
Some types of peripheral circuits, such as redundancy circuits, are used primarily to increase the yield or reduce the cost of manufacturing the chips. Other types of peripheral circuits, such as mode control circuits, timing adjustment circuits, and logic change circuits, are used to modify or control the memory operation. For example, a timing adjustment circuit may be used to either reduce or increase the delay time of one signal with respect to a second signal, in order to provide for synchronous operation.
In fabricating integrated circuits such as DRAMs, it is often desirable to make design changes at a late stage in the fabrication process. While it is possible to redesign by changing the masks and reprocessing, such custom fabrication may be too expensive and time consuming especially if only a small quantity of chips is being produced. Another approach is, for example, to include alternative circuits in the original design and then use a control circuit to "select" one of the alternative circuits later in the fabrication process by using fuses to enable a selected circuit and disable a nonselected circuit.
As used in the prior art, such control circuits have the disadvantage of drawing power even in a nonselected state. It is therefore desirable to provide a programmable control circuit that draws essentially no power to selectively enable or disable such alternative circuits.